In an alternate universe, Empress Celestia is the sole ruler of Equestria - later to be surpassed by her daughters; her heirs, Princesses Sunset and Twilight.
Nightmare Moon had long since banished to the moon, but even for such an exile, there's only so much a moon banishment can do. Her body may be chained, but not her spirit.
Takes place years before Tough Love
Really liking how this AU is being built.
I can't wait for more
If Sunset and Twilight are like their canonical selves Celestia would be grateful for them to sleep. Sunset would probably be challenging her on everything and Twilight would be a steady stream of questions. If Celestia is the nurturing mother first and Empress second her children would view her as such and would not fear her in a way that they would rush to bed every night. especially given the age that they're at.
What would make more sense (for me as a parent) is Twilight and Sunset scrambling to bed because they get to hear bedtime stories, because I would imagine Celestia would have some pretty epic ones. It would also explain why Twilight loves books, because it's when she probably feels the closest to her mother.